The Rogowski Coil was invented by Chattock in the 1880's (Chattock A. P. On a magnetic potentiometer Philos Mag., 1887 pp 94-96). It is a flexible solenoid with a co-axial return conductor and is used in non-intrusive manner to sense the magnetic field generated by a current carrying conductor by encircling said conductor. The output of the Rogowski Coil is a voltage proportional to the derivative or rate of change of the current being measured. Means need to be provided to integrate this voltage output to create a voltage output proportional to the current being measured.
Based on Ampere's theorem, if a solenoid of length 1 having N turns of area A, and a pitch of n=N/1, encircles the current carrying conductor perpendicularly with a radius R=1/2π centred around said conductor, the magnetic field B at the centre of each turn is given by B=μ0i(t)/(2πR). Provided that A<<R2 the flux per turn is AB(t) which leads to the familiar equation for the output of a Rogowski coil being e(t)=−Nd(AB(t)/dt=−μ0nAdi(t)/dt.
The Rogowski coil only integrates the rate of change of flux density B correctly if the coil layout is closed.
Any deviation from this closed loop results in an incomplete line integral, as defined by Ampere's Theorem, and therefore, a degraded accuracy in measurement of the current.
The present state of the art flexible Rogowski coils tend to use butt joins given the nature of the loop closing coupling designs resulting in erroneous measurements. However, other joint designs are also used e.g., right angle joints and parallel joints. By winding extra turns at the butt joint interface improvements to accuracy are possible.
Industrial International Safety Standards e.g., IEC 61010 prescribe minimum creepage and clearance distances which further exacerbate the situation with regard to measurement performance.
The wire contained in a Rogowski coil is generally provided with an insulated covering or coating. Functional insulation for safety purposes as described in safety standards (e.g. EN61010-1:2001 Clause 6.9.1) comprise that the following shall not be used as insulation for safety purposes; (1) materials which can be easily damaged (e.g. physically damaged) for example lacquer, enamels, oxides, anodic films, and (2) non-impregnated hygrospic materials (for example preferably the wire is provided with functional insulation for safety purposes and preferably as described in appropriate safety standards).
It is an aim of the present invention to overcome at least one problem associated with the prior art whether referred to herein or otherwise.